In the prior art, spread spectrum communication is known as a communication providing high security. In a transmitter for spread spectrum communication, for example, digital data to be transmitted is spread over a wide frequency band by using a spreading code having a prescribed length, and then transmitted. In a receiver for spread spectrum communication, by using a code identical with the spreading code of the transmitter, the correlation between this spreading code and a received spread signal is determined and a correlation output peak is obtained. It is usually called a despreading demodulation to determine the correlation, to obtain a sequence of correlation peaks, and to retrieve the original digital signal based on this. In this despreading demodulation, there are two methods. In one method, after detecting a phase difference (synchronization recognition) between local signal generators respectively within the transmitter and the receiver based on time information of the correlation output from a correlator, the despreading demodulation is performed by obtaining the sequence of correlation peaks. In the other method, the despreading demodulation is performed by obtaining the sequence of correlation peaks directly from the correlator. As for the correlator used to detect the correlation, for example, a surface acoustic wave (SAW) device such as a surface acoustic wave convolver or a matched filter may be used.
In order to realize a spread spectrum communication with high quality, it is desirable to increase the length of the spreading code. It is known that by increasing the length of the spreading code, a process gain which is also an index of a suppression level of interference waves in the spread spectrum communication is improved.
However, when the length of the spreading code is increased, an interaction length to obtain the correlation is increased, resulting in an increase in size of the surface acoustic wave device, and various other problems. For example, in the surface acoustic wave convolver, a longitudinal length (interaction length) L of a convolution region required to obtain the correlation output is expressed by the following equation (1).L=N×V/Rc  (1)
where, N is the length (bits) of a spreading code, V is the velocity (m/s) of a surface acoustic wave, and Rc is the chip rate (cps).
In the next-generation mobile communication system such as an IMT-2000, etc., the length of a short code used for an initial synchronization acquisition is 256 bits, and the chip rate is 4 Mcps. Furthermore, since the velocity of surface acoustic wave of a 128 degree rotation Y cut -X direction propagation LiNbO3 which is a typical surface acoustic wave substrate, is 4000 m/s, when a surface acoustic wave convolver is manufactured by using such a piezo-electric substrate, an interaction length L will be 256 mm according to the above equation (1). Generally, when the size of the piezo-electric substrate reaches 100 mm or larger, since the manufacture of the piezo-electric substrate itself is technically difficult, it is very difficult to obtain such a piezo-electric substrate, and even when it is obtained, it will be very expensive.
On the other hand, the size in an orthogonal direction (hereinafter, referred to as a width direction) to the above-mentioned longitudinal direction may be sufficient if it is in the order of several hundred λ, (λ is the wavelength of the surface acoustic wave) although it depends on a type or a structure of an interdigital electrode. In this case, the shape of the piezo-electric substrate required to manufacture the surface acoustic wave convolver will be very elongate shape. Since most of the shapes of the piezo-electric wafers available in the market are circular, in order to cut out a piezo-electric substrate required for the surface acoustic wave convolver without waste, it is necessary not only its size is smaller than that of the piezo-electric wafer but also the lengths in the ongitudinal direction and in the width direction are not different to a great extent. It is an important problem which directly reflects on the manufacturing cost of the surface acoustic wave convolver, how many piezo-electric substrates can be cut out from one sheet of piezo-electric wafer.
Accordingly, when the shape of the piezo-electric substrate is very elongate, since it is impossible to cut out many piezo-electric substrates from one sheet of piezo-electric wafer, there is a problem that the manufacturing cost is increased remarkably. Furthermore, when the shape of the piezo-electric substrate is very elongate, there is another problem that the mechanical bending strength becomes very weak, and the handling is difficult.
Furthermore, a problem also arises in an optical mask and an exposing device required for forming an electrode pattern on the piezo-electric substrate by a photolithography technique or the like. Specifically, the optical mask can be manipulated, at the present time, for an electrode pattern of about 130 mm or less, and it is impossible to manipulate for the electrode pattern exceeding this limit. Also, since usual exposing devices are designed to form an electrode pattern less than 100 mm, a large modification is required for the electrode pattern of 100 mm or larger.
Furthermore, as one of important applications of the surface acoustic wave convolver, the use as a part of a mobile communication handy terminal may be considered, that is, as a correlator in a CDMA (Code-Division Multiple Access) system using a spread spectrum communication by a DS (Direct Sequence) system. The progress in the recent miniaturizing technique of handy terminal for mobile communication is remarkable, and as an example, a mobile phone having a size of 130 mm (length)×41 mm (width)×25 mm (thickness) has been in practical use. For this reason, when the surface acoustic wave convolver is to be used in the handy terminal, it is a mandatory condition that the size of the surface acoustic wave convolver is small as compared with the handy terminal.
Therefore, in the spread spectrum communication using a long spreading code, it has been essentially impossible to mount an analog correlator such as a surface acoustic wave convolver and a surface acoustic wave matched filter and the like having a size as large as 256 mm on the handy terminal.
The present invention was made in view of the unsolved problems in the prior art, and it is an object to provide a spread spectrum signal processing apparatus and a spread spectrum communication system suitable for communication by using a correlator having a size mountable on a portable equipment in a spread spectrum communication using a long spreading code.